It has been recognised for a long time that fiber-reinforced composite materials, particularly carbon fiber composites have great potential for revolutionising the auto industry. It is well known that composites are very light compared to their metal equivalents, even aluminium, and can be formed into complex shapes that can do the same job as many welded metal stampings.
Composites also have the ability to absorb high amounts of energy during impacts which make them ideal for automotive, rail or civil applications. For example, whereas steel can only absorb up to 20 kilojoules per kilogramme and aluminium approximately 30 kilojoules per kilogramme, carbon composites can absorb up to 80 kilojoules per kilogramme.
In addition, unlike metallic structures, the crushed material has very little residual strength after it has absorbed the energy. Instead, the composite material is essentially transformed into small pieces of debris and loosely connected fibres after it has been crushed which means that less space is required than in an equivalent metal structure. This is because in a metal structure space must be provided in designated crumple zones to accommodate the buckled metal.
There is, therefore, a significant incentive to using composite materials such as carbon fiber composites in mass production vehicles. However, to date they have only been used in very limited applications such as top-end sports cars, motor sport and small, non-critical parts of mass produced cars.
Two significant current disadvantages of composites is that they are relatively costly and have long manufacturing cycle times. However, a significant barrier which still remains to their widespread use in the automotive industry is the ability to be able to model their performance in an impact. This is of course essential to be able to do in order to design vehicles which are as safe as possible and which will behave in a predictable way in the event of a crash. Although crash performance testing can be carried out by building prototypes, this is extremely expensive and is only practically feasible in the latter stages of design to prove the basic design and calibrate restraint systems. During the earlier stages of design of vehicles made from metal, finite element analysis is used to model the behaviour and interaction of the various metal parts and to predict their performance in the event of an impact. This means that designs can be proposed, tested and modified using computer modelling with much less reliance on producing and testing expensive prototypes.
However, this approach does not currently work for crushable materials such as composites. The reason for this is that composites absorb energy by a very different mechanism to metallic structures. Metallic structures absorb energy by plastic folding of the metal, initiated by local buckling of the material, which can be characterised by a stress vs strain curve to good effect. At limit, final failure, which may be tearing or brittle fracture, results in the element being unable to transfer load, although its initial volume is essential unchanged.
On the microscopic scale however some materials such as composites absorb energy by local crushing of the material, by matrix cracking, fiber buckling and fracture, frictional heating etc. Viewed on a macro scale, the material is essentially crushed or consumed by the impact on a continuous basis, and the volume of the material is reduced as the structural material is turned to debris.
It is widely recognised in the art that no satisfactory way of modelling the crush performance of composite materials exists. Existing finite element analysis techniques tend to treat elements of composite by treating the whole element or separate layers thereof as maintaining their integrity until the appropriate failure stress value is reached, whereafter the element or layer is simply deleted from the analysis or the element or layer is deleted from the analysis in a predefined period. In a typical example, this might result in the element being deleted with only 5% of its original edge length compressed. The conventional finite element calculations essentially cannot deal with very large changes in volume and therefore catastrophically fail the element where in reality the unimpinged volume of material still had a significant capacity to absorb energy. This has the effect that the results of analysis based on such techniques do not correlate satisfactorily with actual experimental results such that they cannot be relied upon to predict the performance of structures e.g. automotives in the event of an impact.
This is clearly a serious drawback of conventional techniques and in practice means that composite materials are not used or in the few cases where they are used, either the structure must be sufficiently over-engineered to ensure the required minimum level of performance, or extensive prototyping and testing is needed in order to assess performance, which is of course unduly time consuming and expensive.
There exists a need, therefore, to be able to predict reliably the performance of composite materials during an impact.